cmd/yacc: allow leading underscore in token name

Fixes #4037.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6488093

src/cmd/yacc/units.y

@@ -78,7 +78,7 @@ var vflag bool
 
 %token	<vval>	VAL
 %token	<vvar>	VAR
-%token	<numb>	SUP
+%token	<numb>	_SUP // tests leading underscore in token name
 %%
 prog:
 	':' VAR expr
@@ -159,7 +159,7 @@ expr3:
 
 expr2:
 	expr1
-|	expr2 SUP
+|	expr2 _SUP
 	{
 		xpn(&$$, &$1, $2)
 	}
@@ -236,13 +236,13 @@ loop:
 		return '/'
 	case '¹', 'ⁱ':
 		yylval.numb = 1
-		return SUP
+		return _SUP
 	case '²', '⁲':
 		yylval.numb = 2
-		return SUP
+		return _SUP
 	case '³', '⁳':
 		yylval.numb = 3
-		return SUP
+		return _SUP
 	}
 	return int(c)
 

src/cmd/yacc/yacc.go

@@ -988,7 +988,7 @@ func gettok() int {
 
 func getword(c rune) {
 	tokname = ""
-	for isword(c) || isdigit(c) || c == '_' || c == '.' || c == '$' {
+	for isword(c) || isdigit(c) || c == '.' || c == '$' {
 		tokname += string(c)
 		c = getrune(finput)
 	}
@@ -1338,7 +1338,7 @@ loop:
 				if j >= max {
 					errorf("Illegal use of $%v", j)
 				}
-			} else if isword(c) || c == '_' || c == '.' {
+			} else if isword(c) || c == '.' {
 				// look for $name
 				ungetrune(finput, c)
 				if gettok() != IDENTIFIER {
@@ -3090,7 +3090,7 @@ var peekrune rune
 func isdigit(c rune) bool { return c >= '0' && c <= '9' }
 
 func isword(c rune) bool {
-	return c >= 0xa0 || (c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z')
+	return c >= 0xa0 || c == '_' || (c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z')
 }
 
 func mktemp(t string) string { return t }